The present invention relates to bearing materials comprising aluminium alloys bonded to a strong backing material.
Highly rated internal combustion engines have conventionally used crankshaft bearings comprising either a copper based alloy or an aluminum based bearing alloy bonded in some manner to a strong backing or substrate material, such as steel for example. The actual working surface of the bearing alloy, i.e. that surface which faces the engine crankshaft journal surface, has also been provided with a so-called overlay coating which is a thin coating of a relatively softer metal alloy such as lead-tin, lead-tin-copper or lead-indium for example. The purpose of the overlay coating is to provide conformability and dirt embeddability properties to the bearing. Conformability is that property of a bearing that allows the bearing to accommodate slight mechanical misalignments between the bearing and shaft surfaces and is a measure of the ability of the overlay alloy to distribute the applied load. Dirt embeddability is that property that allows debris particles in the lubricating oil to be embedded in the soft overlay alloy without causing damage such as scoring of the shaft. While the technical advantages of overlay coated bearings are not disputed, they have the significant disadvantage of being expensive to make due to the overlay generally being deposited by electroplating, which is a relatively very labor intensive process.
Manufacturers of motor vehicles are more frequently asking for bearings that do not have overlay coatings, as they are cheaper to buy. However, some engines while not possessing a particularly high specific output, due to their design, impose high loads on the crankshaft bearings or possess particularly thin oil films between the bearing and shaft journal and are consequently prone to xe2x80x9cscuffingxe2x80x9d of the bearing surface. Scuffing is where metal to metal contact between the crankshaft journal surface and the bearing surface occurs, i.e. the oil film at the point of contact is ruptured allowing metal to metal contact, without actual seizure and consequent failure of the bearing. However, while overlay coated bearings are especially scuff resistant, most of the conventional copper and aluminum based alloys are relatively poor in terms of scuff resistance. The ability to withstand scuffing is a measure of the conformability of the alloy. In contrast to scuffing, seizure is related to lack of compatibility of the alloy.
One known material comprising: aluminium-6 weight % tin-1 weight % copper-1 weight % nickel has good scuff resistance but has a relatively low fatigue strength and toughness in the non-overlay plated condition, which renders it unsuitable for more modern highly rated engines. The low fatigue strength and toughness is a reflection of the low ductility of this alloy.
To cope with the stresses imposed by modern engines, an alloy having significantly improved mechanical properties, namely, tensile strength (15%); hardness (15%); and fatigue strength (16%) than one of the strongest known aluminum bearing alloys comprising: aluminum-12 weight % tin-4 weight % silicon-1 weight % copper that is in a solution heat treated form, is required. While the strength of this alloy could be raised by increasing the copper content, it is difficult and expensive to make by the conventional production method of casting billets, rolling to size and roll-pressure bonding to steel due to the small size reductions that are possible at each rolling pass before annealing heat treatment is required.
GB-A-2271779 describes an aluminum/tin/silicon bearing alloy which may further comprise at least one of the elements Mn, Mg, V, Ni, Cr, Zr, and/or B at between 0.1 and 3.0 weight % per element. In addition to these elements, the alloy further contains 0.2 to 5.0 weight % Cu, 0.1 to 3.0 weight % Pb, 0.1 to 3.0 weight % Sb and 0.01 to 1.0 weight % Ti as additional alloying elements. It is explained that if the content of the optional elements Mn, Mg, V, Ni, Cr, Zr, and B rises above 3.0 weight %, the conformability of the bearing may deteriorate and workability of the bearing alloy can be degraded.
GB-A-2266546 is also concerned with aluminum-based bearing alloys similar to GB""779 described above. In this case, the alloy also preferably includes at least one or two further elements comprising: from 0.2 to 5.0 weight % Cu, from 0.1 to 3.0 weight % Pb, from 0.1 to 3.0 weight % Sb, Mn, Mg, V and Ni and 0.01 to 1.0 weight % Ti, with the total amount of Mn, Mg, V and Ni being in the range from 0.01 to 3.0 weight %.
However, alloys made according to the teachings of the above two documents are virtually unprocessable by the normal production methods of casting and rolling followed by roll-pressure bonding due to lack of ductility and brittleness of the alloys. This is the case when the alloying element contents are a small fraction of those quoted.
The present invention provides a bearing that includes an aluminum-based bearing alloy having greater strength and scuff resistance than known bearing alloys while retaining ease of manufacture.